Reducing electrical resistance and threshold current is of primary importance in producing highly efficient and reliable vertical cavity surface emitting laser (VCSEL) devices.
The electrical resistance of a VCSEL can be lowered by reducing the resistance of the materials used to fabricate the VCSEL. The physical structure of the VCSEL can also be optimized to reduce resistance by altering the width and length of the conduction path.
A large portion of the overall electrical resistance of the VCSEL may be attributed to the semiconductor distributed Bragg reflectors (DBR) used as the input and output mirrors of the laser cavity. One means of reducing resistance of the DBR components is to replace the top semiconductor DBR mirror with a dielectric thin film DBR. Structurally, this situates a dielectric thin film close to the active region of the device. This approach raises concern because of the difference in coefficients of thermal expansion between the dielectric thin film and the other materials used in the VCSEL. The difference in coefficients of thermal expansion causes strains between the materials and can complicate fabrication processes and device operation.
Threshold current may be reduced by reducing the diameter of the VCSEL aperture. A smaller diameter VCSEL aperture has a relatively lower threshold current when compared to devices with larger diameter VCSEL apertures. VCSEL aperture diameter is largely dependent on the means by which the current confining aperture is fabricated. Small diameter apertures are often difficult to fabricate with ion implantation techniques because of proton straggle. The phenomena of “straggle” results from the scatter of protons through the crystal lattice of conventional semiconductor DBRs. Smaller devices also have more critical alignment tolerances, which, in turn, are dependent on available photolithographic processes.
Two exemplary patents which disclose VCSEL devices are Mori et al., U.S. Pat. Nos. 5,537,666 and 5,587,335. Mori et al. teach the fabrication of laser devices by the sequential formation of semiconductor layers, including a multi-layer semiconductor mirror, a cladding, and an active layer on a substrate through organic metal vapor growth processes. However, Mori et al. do not address the problems of VCSELs beyond those solutions discussed above.
As a result, there remains a need for processes and resulting devices that reduce electrical resistance and threshold current.